76 STAKMAN 



shade trees, respectively. The white pine bhster rust fortunately does not 

 spread from pine to pine but only from pine to currants and gooseberries, 

 from which it spreads back to white pine. The disease has been kept in check 

 in certain areas of the United States, therefore, by destroying susceptible 

 currants and gooseberries in the vicinity of valuable stands of white pine. 

 This would be virtually impossible if the rust spread directly from pine to 

 pine, for the difficulty of controlling the disease in forests is obvious. The 

 so-called Dutch elm disease has destroyed large numbers of the most beauti- 

 ful elms in certain localities in northeastern United States. The only known 

 method of preventing its spread is to find and destroy infected trees. No 

 cure has been found, so that it is a matter of sacrificing some trees in the 

 hope of protecting others. What the final outcome will be no one knows. 

 Oak wilt, probably a native of the United States, has killed large numbers 

 of oaks during the past few years, and the final outcome of its ravages can- 

 not be predicted. 



Obviously there are plant public-health problems in connection with the 

 prevention of epidemics of forest, shade, and orchard trees and many other 

 kinds of plants, because individual effort alone often is ineffective. We still 

 need to learn far more about the pathogens of wild and cultivated plants 

 because many of them are equally at home on both but undetected on the 

 wild. We need to learn far more about the world-wide distribution of patho- 

 gens and their parasitic races in order that we may know where danger 

 lurks and try to avert it by quarantines, eradication campaigns, and other 

 appropriate control measures. We still pay a high price for permitting man to 

 distribute pathogens from one part of the world to another. 



Even after a half century of fairly intelligent effort to control them, plant 

 diseases still cost the people of the United States upwards of 3 billion dollars 

 a year, and this includes principally diseases in the traditional sense, those 

 caused by fungi, bacteria, nematodes, viruses, and a few parasitic seed plants. 

 Many diseases still cannot be controlled at all, some are controlled very 

 imperfectly, and some are controlled only by expensive procedures that add 

 greatly to the cost of production. 



Most major diseases of cultivated crops can become epidemic under favor- 

 able conditions, and it is a continual fight to keep them within bounds. The 

 first problem is to understand them and their potentialities. This is not al- 

 ways easy because most diseases are caused by insidious and shifty micro- 

 scopic organisms, or by ultramicroscopic viruses, each species usually com- 

 prising hundreds or thousands of parasitic strains, usually designated as 

 physiologic races. To find and identify these innumerable races and to deter- 

 mine their effects on thousands of varieties of economic plants is a herculean 

 task in itself, especially since new races are continually being produced and 

 are being disseminated by wind, insects, and man. As none of these agents 

 of dissemination are noteworthy for respecting international boundaries, epi- 



